Field of the Disclosure
The disclosure relates to high power fiber laser systems (“HPFLS”) operative to emit radiation at main and parasitic wavelengths. More particular, the disclosure relates to a gain block with Nd-doped fibers operative to generate/amplify radiation at the desired wavelength while limiting gain at a parasitic wavelength. The disclosure also relates to a gain block with Nd-doped fibers operative to generate high-order harmonics.
Discussion of the Prior Art
Development and power scaling of fiber laser systems emitting light in a 880-960 nm luminescence range have been recently intensified because these systems find a variety of industrial applications. For example, such systems may function as a powerful pump for Yb-doped fiber laser. Alternatively, these fiber laser systems are used for realization of second harmonic generation—a nonlinear optical process associated with the generation of the double frequency and, therefore, half the wavelength, i.e., 450-470 nm. The laser systems operating in this range are known as “blue” lasers.
It's well-known that 4F3/2-4I9/2 transition of neodymium ion corresponding to a 880-960 nm luminescence spectral range terminates at the ground state. Therefore, the signal in this spectral range is subjected to the ground state absorption, and as a result high population inversion is required to achieve a reasonable gain in this spectral range leading to extremely high gain around a parasitic 1060 nm wavelength. Two most common approaches have been traditionally used to solve this problem.
One of the known approaches is based on reducing the concentration of active ions in fiber and the length of active fiber. Implementation of this approach usually requires the use of fibers with very large core diameters for efficient absorption of pump radiation. This in turn entails the substantial increase in gain of the higher order modes. It should be noted that effects of high parasitic gain in 1060 nm range and growth of higher order modes amplification are superimposed on each other and lead to catastrophic growth of gain for higher order modes at 1060 nm. This limitation may be overcome by reducing the core/cladding refractive index difference and inducing bend losses for higher order and even fundamental modes in the long-wavelength spectral range.
Another widely used in practice approach is based on the suppression of the fundamental mode of active fiber in the long-wavelength range through the use of w-profile active fiber. Typically, this approach requires the use of very small core diameters of the active fiber. This entails the use of high concentration of active ions necessary to have reasonable pump absorption and, as a consequence, the high parasitic gain in 1060 nm spectral range that can reach hundreds of dB. It means that the suppression in a w-profile active fiber should exceed hundreds of dB, which is extremely difficult to implement or monitor. Besides, such a high concentration of neodymium ions would result in reduction of pump conversion efficiency. In addition to that compression of mode field diameter corresponding to a small core diameter would lower the threshold of nonlinear phenomena. All of the above mentioned reasons hinder the realization of high-performance and high-power neodymium fiber laser in the 900 nm range.
In summary, the gain block based on a Nd-doped fiber is characterized by amplification at respective signal and parasitic wavelengths. The gain at parasitic wavelengths in a 1060 nm range at the desired value of the gain at signal wavelength in a 900 nm range is a function of the overall length of the Nd-doped fiber and concentration of active ions. Typically, with the increase of fiber length and concentration, the gain in the parasitic range also increases.
The pump absorption in gain blocks in a Nd-doped fiber is also proportional to the length of the Nd-doped fiber and concentration of active ions. Thus, increasing pump absorption by having the greater length and higher Nd ions concentration inevitably leads to the growth of parasitic amplification.
A need therefore exists for a gain block based on a Nd-doped fiber and a method for manufacturing the block in which the pump light absorption is improved without the necessity for the increased dopant concentration and the increased active fiber length.
Another need exists for a high performance and high-power fiber laser system provided with the neodymium fiber gain block.
Still a further need exists for a fiber laser system based on the neodymium-doped fiber gain block for generating a second or higher harmonic of the gain block's radiation.